R.M.
Stauffer, A.M. Thompson, and G.S. Young, 2016
Tropospheric
ozonesonde profiles at long-term U.S. monitoring sites: 1. A climatology based
on self-organizing maps
Journal
of Geophysical Research – Atmospheres. 121, 1320-1339
Abstract
Sonde-based climatologies of tropospheric ozone (O3) are vital
for developing satellite retrieval algorithms and evaluating chemical transport
model output. Typical O3 climatologies average measurements by
latitude or region, and season. A recent analysis using self-organizing maps
(SOM) to cluster ozonesondes from two tropical sites found that clusters of O3
mixing ratio profiles are an excellent way to capture O3 variability
and link meteorological influences to O3 profiles. Clusters
correspond to distinct meteorological conditions, e.g., convection, subsidence,
cloud cover, and transported pollution. Here the SOM technique is extended to
four long-term U.S. sites (Boulder, CO; Huntsville, AL; Trinidad Head, CA; and
Wallops Island, VA) with 4530 total profiles. Sensitivity tests on k-means
algorithm and SOM justify use of 3 × 3 SOM (nine clusters). At each
site, SOM clusters together O3 profiles with similar tropopause
height, 500 hPa height/temperature, and amount of tropospheric and total
column O3. Cluster means are compared to monthly O3
climatologies. For all four sites, near-tropopause O3 is double
(over +100 parts per billion by volume; ppbv) the monthly climatological O3
mixing ratio in three clusters that contain 13–16% of profiles, mostly in
winter and spring. Large midtropospheric deviations
from monthly means (−6 ppbv, +7–10 ppbv O3 at 6 km)
are found in two of the most populated clusters (combined 36–39% of profiles).
These two clusters contain distinctly polluted (summer) and clean O3
(fall-winter, high tropopause) profiles, respectively. As for tropical profiles
previously analyzed with SOM, O3 averages are often poor
representations of U.S. O3 profile statistics.